Metal-air batteries are electrochemical cells wherein the oxidation of a metal anode by an oxidizing agent, such as air, creates a flow of electrical current. As such, metal-air batteries may also be considered to be a type of a fuel cell wherein the metal of the anode is considered to be the fuel.
A generalized metal-air battery includes an anode which is comprised of, or contains, an electrochemically active metal which is oxidized in the operation of the battery to generate electrical current. The metal-air battery further includes a cathode, and in the operation of the battery, oxygen is reduced at the cathode. Typically, the cathode is fabricated from an electrically conductive, air-permeable material such as a carbon fabric, porous metal or the like. The cathode may include a catalytic material therein to facilitate the reduction of oxygen, and such catalytic materials may include noble metals such as platinum and its compounds and alloys. The metal-air battery will also include an electrolyte which is ionically conductive and which is in contact with the anode and cathode. As is typical in batteries, a porous body of separator material may be disposed between the anode and cathode. As will be described in detail hereinbelow, such separators may comprise porous polymeric membranes as well as porous, fibrous materials such as glass, ceramic or cellulosic materials.
Metal-air cells typically have a high energy density as compared to other types of electrochemical cells. Heretofore, metal-air batteries have been fabricated utilizing relatively light metals such as magnesium, aluminum and zinc as electrochemically active anode materials. However, batteries utilizing magnesium or aluminum-based anodes have been found to exhibit problems of stability due to chemical reactions between the metals and electrode components and electrolytes. Zinc-air batteries have been found to be relatively stable when used in conjunction with alkaline electrolytes, and such batteries are in commercial use. Zinc-air batteries have a high theoretical voltage; however, their ultimate energy density is limited by the fact that the zinc comprising the anode undergoes only two-electron oxidation. The theoretical energy density of zinc-air batteries utilizing an alkaline electrolyte is only 489 Wh/Kg based upon the reaction of Zn+2 KOH. Other limitations upon the utility of zinc-air batteries involve the evolution of hydrogen at the anode causing fuel waste and increasing cell resistance. Additives such as mercury and lead may be utilized to retard hydrogen evolution; however, the poisonous nature of these additives does restrict their use. It has also been found that during storage, the electrolytes of zinc-air batteries tend to dry thereby decreasing battery performance. Storage can also cause carbonate formation in the zinc-air battery which can damage separator membranes and compromise cell performance.
As will be appreciated from this discussion, metal-air batteries have many potential advantages over conventional electrical power sources; however, battery structures and materials available to date have not fully exploited the advantages of metal-air battery systems. As will be explained in detail hereinbelow, the present invention provides a novel metal-air battery structure and chemistry which significantly improves upon metal-air battery performance characteristics. These and other advantages of the invention will be apparent from the drawings, discussion and description which follow.